Packaging optoelectronic components and CMOS circuitry using silicon-on-insulator substrates for photonics applications

ABSTRACT

Package structures and methods are provided to integrate optoelectronic and CMOS devices using SOI semiconductor substrates for photonics applications. For example, a package structure includes an integrated circuit (IC) chip, and an optoelectronics device and interposer mounted to the IC chip. The IC chip includes a SOI substrate having a buried oxide layer, an active silicon layer disposed adjacent to the buried oxide layer, and a BEOL structure formed over the active silicon layer. An optical waveguide structure is patterned from the active silicon layer of the IC chip. The optoelectronics device is mounted on the buried oxide layer in alignment with a portion of the optical waveguide structure to enable direct or adiabatic coupling between the optoelectronics device and the optical waveguide structure. The interposer is bonded to the BEOL structure, and includes at least one substrate having conductive vias and wiring to provide electrical connections to the BEOL structure.

TECHNICAL FIELD

This disclosure relates generally to packaging techniques for photonicsapplications and, in particular, to structures and methods forintegrating optoelectronic devices and CMOS (complementary metal oxidesemiconductor) devices for photonics applications.

BACKGROUND

In general, photonics applications implement various functions withregard to light including, for example, generating, emitting,transmitting, modulating, signal processing, amplifying, and/ordetecting/sensing light within the visible and near-infrared portions ofthe electromagnetic spectrum. Various techniques have been developed forimplementing photonics applications. For example, some conventionaltechniques involve co-fabricating optoelectronic devices with CMOSintegrated circuitry to implement photonics systems. The main challengewith these techniques is that the lithography used for photonics isseveral generations behind the most advanced CMOS. Typically, thelithography for photonics is in the range of 130 nm to 90 nm, andtherefore, CMOS circuitry formed based on these design rules provideslimited speed performance, thus limiting the electrical and photonicsI/O speed.

Other conventional techniques for implementing photonics applicationsinclude fabricating dedicated silicon photonics chips with no integratedCMOS. The main problem with these techniques is the lack of integratedCMOS functions and therefore, the lack of analog and digital on-chipcontrols. For example, a ring resonator array with a heater control loopwould be difficult to implement. Another problem with this approach isthat high-speed I/O data communications between silicon photonics chipsand other electronics chips mounted on an application board isimplemented using wire-bond connections to the application board. Thescaling of data communication above 25 Gbit/s with wire bonding isextremely difficult. Moreover, when using wire bonds with optoelectronicchips having optoelectronic components such as laser diodes, there is noroom to install a heat sink on the optoelectronic chips, which iscritical for reliable operation of laser diodes, for example.

SUMMARY

Embodiments of the invention include package structures and methods tointegrate optoelectronic and CMOS devices using SOI(silicon-on-insulator) semiconductor substrates for photonicsapplications.

In one embodiment of the invention, a package structure includes aphotonics package, wherein the photonics package includes an integratedcircuit chip, an optoelectronics device mounted to the integratedcircuit chip, and an interposer mounted to the integrated circuit chip.The integrated circuit chip includes a SOI substrate, wherein the SOIsubstrate has a buried oxide layer, an active silicon layer disposedadjacent to the buried oxide layer, and a BEOL (back-end-of-line)structure formed over the active silicon layer. An integrated opticalwaveguide structure is patterned from the active silicon layer of theintegrated circuit chip. The optoelectronics device is mounted on theburied oxide layer of the integrated circuit chip in alignment with atleast a portion of the integrated optical waveguide structure. Theinterposer is bonded to the BEOL structure of the integrated circuitchip. The interposer includes at least one substrate having a pluralityof conductive through vias and wiring to provide electrical connectionsto the BEOL structure.

In another alternate embodiment of the invention, the package structurefurther includes a second integrated circuit chip, a package interposer,and an application board. The photonics package is mounted to a firstside of the package interposer and the second integrated circuit chip ismounted to a second side of the package interposer, opposite the firstside of the package interposer. The package interposer includeselectrical wiring and conductive through vias to provide electricalconnections between the photonics package and the second integratedcircuit chip. The application board includes an integrated recess formedin one side of the application board. The package interposer is mountedto the application board with at least a portion of the photonicspackage disposed within the integrated recess of the application board.The application board also includes a plurality of thermal vias formedtherein in alignment with the integrated recess. The photonics packageis disposed within the integrated recess of the application board suchthat a backside of the optoelectronics device of the photonics packageis in thermal contact with the plurality of thermal vias.

In yet another alternate embodiment of the invention, the packagestructure further includes a second integrated circuit chip, a packageinterposer having a hole formed through the package interposer, and anapplication board. The second integrated circuit chip is flip-chipmounted to a first side of the package interposer. The photonics packageis mounted to a front side of the second integrated circuit chip anddisposed within the hole of the package interposer. A second side of thepackage interposer is mounted to a first side of the application board.The application board includes a heat sink formed on the first side ofthe application board, and a plurality of thermal vias formed therein inalignment with heat sink. The photonics package is disposed within thehole of the package interposer such that a backside of theoptoelectronics device of the photonics package is in thermal contactwith the heat sink formed on the first side of the application board.

Another embodiment of the invention includes a method to construct apackage structure. The method includes: fabricating an integratedcircuit chip comprising a SOI substrate, wherein the SOI substratecomprises a bulk substrate layer, a buried oxide layer disposed on thebulk substrate layer, an active silicon layer disposed on the buriedoxide layer, and a BEOL structure formed over the active silicon layer,wherein the active silicon layer comprises an integrated opticalwaveguide structure; bonding a first surface of an interposer substrateto the BEOL structure of the integrated circuit chip; forming conductivethrough vias in the interposer substrate in alignment with contact padsof the BEOL structure, and forming contact pads on a second surface ofthe interposer substrate; removing the bulk substrate layer; forming oneor more inverted pad structures through the buried oxide layer to buriedpads in the BEOL structure; forming solder bumps on the contact pads ofthe interposer substrate; and mounting an optoelectronics device to theintegrated circuit chip such that the optoelectronics device iselectrically connected to one or more of the inverted pad structures andsuch that the optoelectronics device is in contact with a portion of theburied oxide layer and in alignment with at least a portion of theintegrated optical waveguide structure.

These and other embodiments of invention will be described or becomeapparent from the following detailed description of embodiments, whichis to be read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic cross-sectional side view of a package structure tointegrate optoelectronic and CMOS devices using a SOI semiconductorsubstrate for a photonics application, according to an embodiment of theinvention.

FIGS. 2A, 2B, and 2C are schematic views that illustrate a method forelectrically and optically coupling an optoelectronics device to a SOIsemiconductor chip, according to an embodiment of the invention.

FIGS. 3A, 3B, and 3C are schematic views that illustrate a method forelectrically and optically coupling an optoelectronics device to a SOIsemiconductor chip, according to another embodiment of the invention.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, and 4J schematicallyillustrate a method for fabricating a photonics package structure,according to an embodiment of the invention, wherein:

FIG. 4A is a cross-sectional view of a photonics package structure at anintermediate stage of fabrication wherein a SOI semiconductor chip andinterposer are bonded together using an adhesive layer, according to anembodiment of the invention;

FIG. 4B is a cross-sectional schematic view of the package structure ofFIG. 4A after filing via holes of the interposer with metallic materialto form conductive through vias;

FIG. 4C is a cross-sectional schematic view of the package structure ofFIG. 4B after removing a bulk substrate layer of the SOI semiconductorchip;

FIG. 4D is a cross-sectional schematic view of the package structure ofFIG. 4C after patterning a layer of metallic material on a surface ofthe interposer substrate to form contact pads that are electricallyconnected to the conductive vias of the interposer;

FIG. 4E is a cross-sectional schematic view of the package structure ofFIG. 4D after forming a photoresist mask and etching a recess in the SOIsemiconductor chip down to a buried pad in a BEOL structure of the SOIsemiconductor chip;

FIG. 4F is a cross-sectional schematic view of the package structure ofFIG. 4E after removing the photoresist mask and depositing a seed layerwhich lines exposed surfaces within the recess;

FIG. 4G is a cross-sectional schematic view of the package structure ofFIG. 4F after depositing and patterning a layer of photoresist materialto form a photoresist mask which is used to cover a portion of the seedlayer on the buried oxide layer and to expose the recess;

FIG. 4H is a cross-sectional schematic view of the package structure ofFIG. 4G after filling the recess with metallic material;

FIG. 4I is a cross-sectional schematic view of the package structure ofFIG. 4H after removing the photoresist mask and etching back themetallic material down to the buried oxide layer; and

FIG. 4J is a cross-sectional schematic view of the package structure ofFIG. 4I after forming solder balls on the contact pads of theinterposer.

FIG. 5 schematically illustrates a high-level conceptual packageframework to implement an optical transceiver system, according to anembodiment of the invention.

FIG. 6 is a schematic cross-sectional side view of a package structureto implement an optical transceiver system based on the conceptualframework of FIG. 5, according to an embodiment of the invention.

FIG. 7 is a schematic cross-sectional side view of a package structureto implement an optical transceiver system based on the conceptualframework of FIG. 5, according to another embodiment of the invention.

DETAILED DESCRIPTION

Embodiments will now be described in further detail with regard topackage structures and methods to integrate optoelectronic and CMOSdevices using SOI semiconductor substrates for photonics applications.It is to be understood that the various layers, structures, and regionsshown in the accompanying drawings are schematic illustrations that arenot drawn to scale. In addition, for ease of explanation, one or morelayers, structures, and regions of a type commonly used to formsemiconductor devices or structures may not be explicitly shown in agiven drawing. This does not imply that any layers, structures, andregions not explicitly shown are omitted from the actual semiconductorstructures.

Furthermore, it is to be understood that the embodiments discussedherein are not limited to the particular materials, features, andprocessing steps shown and described herein. In particular, with respectto semiconductor processing steps, it is to be emphasized that thedescriptions provided herein are not intended to encompass all of theprocessing steps that may be required to form a functional semiconductorintegrated circuit device. Rather, certain processing steps that arecommonly used in forming semiconductor devices, such as, for example,wet cleaning and annealing steps, are purposefully not described hereinfor economy of description.

Moreover, the same or similar reference numbers are used throughout thedrawings to denote the same or similar features, elements, orstructures, and thus, a detailed explanation of the same or similarfeatures, elements, or structures will not be repeated for each of thedrawings. It is to be understood that the terms “about” or“substantially” as used herein with regard to thicknesses, widths,percentages, ranges, etc., are meant to denote being close orapproximate to, but not exactly. For example, the term “about” or“substantially” as used herein implies that a small margin of error ispresent, such as 1% or less than the stated amount.

FIG. 1 is schematic cross-sectional side view of a package structure tointegrate optoelectronic and CMOS devices using a silicon-on-insulatorsemiconductor substrate, according to an embodiment of the invention. Inparticular, FIG. 1 is a schematic cross-sectional side view of a packagestructure 100 comprising a photonics package 110 which includes anassembly of a semiconductor chip 120 with integrated photonicscomponents, an interposer 130, and an optoelectronic device 140. Thepackage structure 100 further comprises an application board 150, and anintegrated circuit (IC) chip 160, wherein the photonics package 110 andthe IC chip 160 are flip-chip mounted to the application board 150 usingan array of solder ball controlled collapse chip connections (C4) 170,for example, or other suitable flip-chip techniques such as micro-C4 orcopper pillars, etc.

The semiconductor chip 120 comprises an insulating layer 121, an activesilicon layer 122, and a BEOL (back-end-of-line) structure 123. Theactive silicon layer 122 is patterned and processed to form activedevices 124 and one or more of an optical silicon waveguide structure126 (e.g., single mode silicon-on-insulator waveguides). In oneembodiment of the invention, the semiconductor chip 120 is fabricatedstarting with a SOI (silicon-on-insulator) substrate comprising a bulksubstrate (which is removed), a BOX (buried oxide) layer disposed on thebulk substrate, and a thin layer of silicon (SOI layer) disposed on theBOX layer. In one embodiment, the insulating layer 121 in FIG. 1 is aBOX layer, and the active silicon layer is the thin SOI layer formed onthe BOX layer.

The active devices 124 and other semiconductor components formed fromthe active silicon layer 122 comprise active circuitry to implement oneor more photonic applications. For example, the active circuitry mayinclude optical receivers, optical transmitters or optical transceivercircuits, and other active or passive circuit elements that are commonlyused to implement photonic systems. The BEOL structure 123 includestransmission lines and other interconnect structures that areimplemented using a series of interconnected metallic traces andconductive vias 125 which are formed within various alternatingconductive and insulating/dielectric layers of the BEOL structure 123.The BEOL structure 123 provides a network of interconnects to connectactive circuitry and other components formed in the active layer 122.Furthermore, the BEOL structure 123 comprises a plurality ofbonding/contact pads 127 such as, for example, ground pads, DC powersupply pads, input/output pads, control signal pads, associated wiring,etc., that are formed as part of a final metallization level of the BEOLstructure 123.

The interposer 130 comprises a substrate 132, conductive thru vias 134,and a pattern of bonding pads/wiring 136 formed on one surface thereof.In one embodiment of the invention, the interposer substrate 132 isformed of a high-resistivity material such as glass, high-resistivitysilicon (HR-Si), or other suitable insulating materials having aresistivity in a range of about 100 Ohm·cm to about 1000 Ohm·cm orgreater. Materials such as glass or HR-Si are desirable materialsbecause they have a coefficient of thermal expansion that is the same orsimilar to the materials of the semiconductor ship 120, which serves toprevent cracking or chip delamination due to thermal expansion andcontraction over time. In one embodiment of the invention, theinterposer 130 has a thickness of at least 300 um to allow reliablemechanical support.

The conductive through vias 134 (e.g., TGVs (through glass vias) or TSVs(through silicon vias) provide electrical connections between thebonding pads/wiring 136 of the interposer 130 and the bondingpads/wiring 127 of the BEOL structure 123. The conductive through vias134 form part of the electrical wiring and interconnects that areutilized for supplying/distributing DC power to the semiconductor chip120 from power supply lines on the application board 150, and forrouting low frequency control signals as well as high-frequency I/Osignals, for example, between the application board 150 and thesemiconductor chip 120. For high-speed data communication, the use of alow-loss, high-resistivity (>1K Ohm·cm) interposer substrate material ishighly desirable to decrease the energy per bit dissipated whentransmitting I/O signals through the interposer 130.

The semiconductor chip 120 with integrated photonics components isbonded to the interposer 130 using an adhesive layer 112. The interposer130 and semiconductor chip 120 can be assembled together on a waferscale or chip scale level. In particular, for a wafer scaleimplementation, a wafer scale interposer and full semiconductor waferare first bonded together, and then the assembly is diced into discretecomponents. In the wafer scale implementation, the size (footprint) ofthe interposer 130 and the semiconductor chip 120 would be the same.Furthermore, in the wafer scale implementation, the size of thesemiconductor chip 120 can be larger than the maximum reticle size, andas large as the wafer (wafer scale integration). In this regard, in oneembodiment of the invention, the semiconductor chip 120 shown in FIG. 1may comprise a semiconductor wafer.

For a chip scale implementation, a semiconductor wafer is diced intoindividual chips (e.g., semiconductor chip 120, FIG. 1), and then theindividual chips are assembled to separate interposers. For the chipscale implementation, an interposer can have a larger footprint sizethan the semiconductor chip, which provides an advantage of allowing theinterposer to spread a larger number of I/Os to the application board,as compared to the wafer scale solution.

As further shown in FIG. 1, the photonics package 110 and the IC chip160 are flip-chip mounted to the application board 150 using C4connections 170, for example, or other suitable flip-chip bondingtechniques. The application board 150 comprises high-speed transmissionlines 154 formed on a surface of the application board 150 to enablehigh-speed communications between the photonics package 110 and the ICchip 160. In one embodiment of the invention, the IC chip 160 comprisesa VLSI digital/analog chip 160 (e.g., a microprocessor, a transceiver,etc.), which can be fabricated using chip technologies such as bulk Si,SiGe, GaAs, InP, etc. While the IC chip 160 could be wire-bonded to theapplication board 150 instead of flip-chip mounted, this mountingtechnique is not as desirable as this results in a decrease from themaximum data transmission speed that could be achieved with flip-chipmounting.

In one embodiment of the invention, the thickness of the BEOL structure123 is about 10 um to about 15 um. In addition, the thickness of theactive silicon layer 122 is about 0.15 um. As noted above, the activesilicon layer 122 is patterned to form the optical waveguide structure126 to transmit light to and from the optoelectronic device 140, whereinthe SOI film is used as a waveguide for the light. The thickness of theinsulating layer 121 (e.g., BOX layer) is typically about 0.15 um whichis about 10× smaller than the wavelength of light that is used by thephotonic devices.

As further shown in FIG. 1, since the thickness of the insulating layer121 may not be a thick enough cladding to prevent light from leaking outof the core of the optical waveguide 126, a thin capping layer 142 isformed on the insulating layer 121 to provide additional cladding. Inone embodiment, the capping layer 142 has a thickness of about 2 um. Inone embodiment, the capping layer 142 is formed of a dielectric materialhaving a dielectric constant that is lower than the dielectric constantof the material forming the waveguide core. For example, the cappinglayer 142 can be formed of SiN or SiO₂, or some other low-dielectricconstant material which is transparent for the operating wavelength oflight.

The capping layer 142 is patterned to expose portions of the underlyinginsulating layer 121 in regions of the semiconductor chip 120 wherephotonic devices are connected to the semiconductor chip 120. Forinstance, as shown in FIG. 1, the capping layer 142 is patterned toexpose a portion of the underlying insulating layer 121 where theoptoelectronic device 140 is mounted in alignment with one end of thesilicon waveguide 126. This enables light to be coupled between theoptoelectronic device 140 and the end portion of the silicon waveguide126 using adiabatic coupling or vertical grating coupling techniques, asdiscussed below.

Furthermore, as shown in FIG. 1, a polymer waveguide 180 (e.g., flexribbon waveguide) is connected to the photonics package 110 by removinga portion of a cladding layer 184 to expose a length portion of an innercore layer 182, and then connecting the exposed portion of the core 182to the insulating layer 121 (e.g., BOX layer). This configurationenables adiabatic coupling of light between the polymer waveguide 180and the silicon waveguide 126. The light from a laser, a modulator, or aphotodiode, etc., can also be coupled adiabatically or through avertical grating coupler.

The illustrative package structure 100 shown in FIG. 1 allows a largeamount of integrated photonics devices (coupler, filter, multiplexer . .. ) to be integrated with VLSI CMOS and external photonics (laser,photodiode . . . ) using 3D assembly (as described below). In thisregard, packaging techniques according to embodiments of the inventionas discussed herein provide flexible package architectures, because suchpackaging structures enable the heterogeneous packaging and integrationof CMOS and photonics devices with external photonics devices. Forexample, the integration of a laser diode within a CMOS chip isextremely challenging since laser diodes are formed with compoundsemiconductors. As such, the heterogeneous package integration of anoptoelectronics device 140 (having laser diodes) with a CMOS chipprovides a more practical approach. It is to be understood that somephotonics functions (e.g., a modulator) can beco-integrated/co-fabricated with CMOS circuitry in a VLSI SOI chip, orotherwise formed as part of a separate external optoelectronic chip thatis packaged with a VLSI SOI chip using techniques as described herein.The package designer will choose the best approach based on performance,size, cost, thermal considerations.

Heat sinks are usually implemented for reliable operation of photonicsdevices. In the package structure shown in FIG. 1, a backside surface ofthe optoelectronics device 140 is exposed so that a cooling member, suchas a heat sink or cooling plate can be thermally coupled to the backsidesurface of the active optoelectronics device 140 for cooling.

In an alternate embodiment of the invention, the interposer 130 may be amulti-layer structure having more than one interposer substrate, whereinthe substrates are mounted to each other and connected using standardbonding techniques. Furthermore, while the illustrative embodiment ofFIG. 1 shows a photonics package structure comprising one semiconductorchip, in an alternate embodiment of the invention, a photonics packagestructure can be fabricated by stacking two or more integrated circuitchips together along with one or more interposer substrates using 3Dpackaging techniques. For example, a second integrated circuit chipcomprising one or more memory arrays or active circuitry performingadditional functions can be included in a photonics package of FIG. 1along with the semiconductor chip 120 in a 3D stacked configuration. Inthis case, second semiconductor chip can be mounted face-to-face, forexample, with the second semiconductor chip having TSVs (silicon throughvias) formed in the backside of the chip to provide electricalconnections to an interposer.

FIGS. 2A, 2B, and 2C are schematic views that illustrate a method forelectrically and optically coupling an optoelectronics device to a SOIsemiconductor chip, according to an embodiment of the invention. Morespecifically, FIG. 2A is a schematic cross-sectional view showing anoptoelectronics device 200 mounted to a SOI semiconductor chip 220, FIG.2B is a lateral cross-sectional view of the structure of FIG. 2A alongline 2B-2B in FIG. 2A, and FIG. 2C is a lateral cross-sectional view ofthe structure of FIG. 2A along line 2C-2C in FIG. 2A.

As shown in FIGS. 2A-2C, the optoelectronics device 200 comprises atleast one ohmic contact 202 formed on one side thereof. Thesemiconductor chip 220 is similar to the semiconductor chip 120 inFIG. 1. However, FIGS. 2A-2C further depict the use of at least oneinverted pad 222 to electrically connect the external optoelectronicsdevice 200 to the semiconductor chip 220. The inverted pad 222 comprisesa conductive via that is formed through the BOX layer 121 usingtechniques discussed in further detail below. The optoelectronics device200 (with the ohmic contact(s) 202) is separately fabricated and thenflip-chip mounted to the semiconductor chip 220 by bonding the ohmiccontact 202 to the inverted pad 222. While only one ohmic contact 202and corresponding inverted pad 222 is shown in FIGS. 2A and 2B, theoptoelectronics device 200 may have two or more ohmic contacts formed onan active surface thereof, which are connected to two or morecorresponding inverted pads formed in the semiconductor chip 220.

The ohmic contact 202 and inverted pad 222 form an electrical contactbetween the external photonics device 200 and the active circuitry(e.g., FET 124) of the semiconductor chip 220. These electrical contactsenable power to be supplied to the optoelectronics device 200 from thesemiconductor chip 220, as well as transmit electrical control signalsor data between the optoelectronics device 200 and the semiconductorchip 200. Since the electrical connections between the active circuitryand the external photonic device 200 are short (about 0.25 um to about10 um), very high efficiency and high bit rate data transfer can beachieved.

As further shown in FIG. 2C, the silicon waveguide 126 has a continuouslength that extends below central portion of the optoelectronics device200 to provide adiabatic coupling of light between the external photonicdevice 200 and an end portion of the silicon waveguide 126. In otherwords, an end portion of the silicon waveguide 126 which is aligned tothe optoelectronics device 200 provides an adiabatic coupler thatcouples light travelling horizontally from the Si waveguide 126 to theexternal optoelectronics device 200. The external optoelectronics device200 comprises an integrated photonic waveguide (not shown) that isdisposed parallel to the photonic waveguide 126 and separated at adistance that is defined by the thickness of the insulating BOX layer121. Since a BOX layer implemented in VLSI SOI technology is muchthinner (˜0.15 um) than the light wavelength (˜1.5 um), the light leaksfrom one waveguide to the other, thus coupling the light between theintegrated and external photonic devices. The adiabatic coupling allowshorizontal coupling of external photonic devices such as, for example,DFB (distributed feedback) laser diodes, as well as integrated photonicdevices such as multiplexers, modulators or filters which are formed aspart of the active circuitry of the semiconductor chip.

FIGS. 3A, 3B, and 3C are schematic views that illustrate a method forelectrically and optically coupling an optoelectronics device to a SOIsemiconductor chip, according to another embodiment of the invention.More specifically, FIG. 3A is a schematic cross-sectional view showingan optoelectronics device 300 mounted to a SOI semiconductor chip 320,FIG. 3B is a lateral cross-sectional view of the structure of FIG. 3Afrom the view point of line 3B-3B in FIG. 3A, and FIG. 3C is a lateralcross-sectional view of the structure of FIG. 3A along line 3C-3C inFIG. 3A.

As shown in FIGS. 3A-3C, the optoelectronics device 300 comprises aplurality of ohmic contacts 302 and 304 formed on one side thereof, andthe semiconductor chip 320 comprises a plurality of correspondinginverted pads 322 and 324, which are electrically connected to thecorresponding ohmic contacts 302 and 304, to electrically connect theexternal optoelectronics device 300 to the semiconductor chip 320. Theoptoelectronics device 300 (with the ohmic contacts 302 and 304) isseparately fabricated and then flip-chip mounted to the semiconductorchip 320 by bonding the ohmic contacts 302 and 304 to the respectiveinverted pads 322 and 324, to form electrical contacts between theexternal optoelectronics device 300 and the active circuitry (e.g., FET124) of the semiconductor chip 320. While only two ohmic contact 302 and304 and corresponding inverted pads 322 and 324 are shown in FIGS. 3Aand 3B, the optoelectronics device 300 may have three or more ohmiccontacts formed on an active surface thereof, which are connected tothree or more corresponding inverted pads formed in the semiconductorchip 320.

As noted above, these electrical contacts enable power to be supplied tothe optoelectronics device 300 from the semiconductor chip 320, as wellas electrical signals to be transmitted between the optoelectronicsdevice 300 and the semiconductor chip 320. Since the connections betweenthe circuit and the external photonic device are short (about 0.25 um toabout 10 um), very high efficiency and high bit rate data transfer canbe achieved.

As further shown in FIG. 3C, and end portion of the silicon waveguide126 comprises a vertical grating coupler 128 which extends below acentral portion of the optoelectronics device 300 to provide gatedcoupling of light between the external photonic device 300 and endportion of the silicon waveguide 126. The vertical grating coupler 128comprises a vertical grating coupler such as a fully etched second-orderwaveguide grating that out-couples optical radiation from the siliconwaveguide 126 at a well-defined angle with respect to the surface normalof the semiconductor chip 320. The vertical grating coupler 128 isconfigured to change the light propagation direction from horizontal inthe Si photonic waveguide 126 to vertical at the grating coupler output128. This optical coupling enables the use of external verticalphotonics devices such as vertical-cavity surface-emitting laser (VCSEL)laser elements or photodiodes, wherein a VCSEL semiconductor laser diodeemits an optical laser beam perpendicular from a surface of theoptoelectronics device 300.

FIGS. 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, and 4J schematicallyillustrate a method for fabricating a photonics package structure,according to an embodiment of the invention. In particular, FIGS. 4A-4Jillustrate a method for assembling a SOI semiconductor chip andinterposer to form a photonics package, such as the photonics package110 of FIG. 1, according to an embodiment of the invention. FIG. 4A is across-sectional view of a photonics package structure 410 at anintermediate stage of fabrication where a SOI semiconductor substrate420 and interposer 430 are bonded together using an adhesive layer 412.

The SOI semiconductor substrate 420 comprises a bulk substrate layer400, a BOX layer 421, an active silicon layer 422, and a BEOL structure423. The SOI semiconductor substrate 420 can be fabricated usingstandard CMOS and VLSI front-end-of-line processing steps to form activecircuitry 424 and one or more silicon waveguides 426. The BEOL structure423 comprises multiple levels of insulating material, via contacts andwiring 425 to interconnect the active circuitry, and to provideinterconnects to a plurality of contact pads 427 that are formed as partof the final metallization level of the BEOL structure 423. The contactpads 427 provide bonding sites for chip-to-interposer connections.

As further shown in FIG. 4A, the interposer 430 comprises a substrate432 having via holes 431 formed through the substrate 432. The via holes431 are aligned to corresponding contact pads 427 of the BEOL structure423 of the SOI semiconductor chip 420. As noted above, the substrate 432may be formed of glass or high-resistivity silicon, for example. In oneembodiment, when the interposer 430 is fabricated using a glasssubstrate 432, the via holes 431 (through glass vias) are formed in thesubstrate 432 prior to bonding the substrate 432 to the semiconductorchip 420 using the adhesive layer 412. In another embodiment, when theinterposer 430 is fabricated using a high-resistivity Si substrate 432,the substrate 432 can be bonded to the semiconductor chip 420 using theadhesive layer 412 prior to forming the via holes 431 (through siliconvias). Once the substrate 432 is bonded to the semiconductor chip 420,the via holes 431 can be etched using a standard photolithographicprocess using a patterned photoresist layer.

The substrate 432 (either glass substrate or HR-Si substrate) can bealigned to the semiconductor chip 420 using TGVs or TSVs that can becentered using dedicated last metal features that are designed into thesemiconductor chip 420 which are not intended for electricalconnections. For example, cross-shaped features can be patterned in thelast metal level of the BEOL structure 423 of the semiconductor chip420, wherein the cross-shaped features are centered within thecorresponding TGVs or TSVs across the die or wafer. As noted above, itis to be understood that such process can be performed for a single chipor the full wafer (wafer scale packaging). For example, with a singlediced chip process, the interposer substrate 432 (glass orhigh-resistivity substrate) can be larger than the semiconductor chip420, thus allowing to spread a large amount of wires between thesemiconductor chip 420 and an application board (e.g., board 150,FIG. 1) through the interposer 430. For wafer scale level packaging,once the full packaging is done and the chips are diced, the interposer430 will have the exact same dimension as the SOI semiconductor chip420.

A next step in the fabrication process includes metalizing the via holes431 to form conductive through vias in the interposer substrate 432. Forexample, FIG. 4B is a cross-sectional schematic view of the packagestructure of FIG. 4A after filing the via holes 431 with metallicmaterial to form conductive through vias, according to an embodiment ofthe invention. The conductive through vias can be fabricated using knowntechniques. For example, a thin barrier layer (e.g., Ta or TiN) can bedeposited to line the exposed surfaces of the substrate 432 in the viaholes 431, followed by the deposition of a thin seed layer 433 (e.g.,copper seed layer). The barrier layer and seed layer 433 can bedeposited using known techniques, such as chemical vapor deposition(CVD) or physical vapor deposition (PVD), etc. The seed layer 433 isthen used as a base layer to grow a layer of metallic material (e.g.,copper) on the seed layer 433 using standard electroplating techniques(e.g., for CMOS backend processes), and fill the via holes 431 withmetallic material to form conductive vias 434 in the substrate 432.

After forming the conductive vias 434 in the interposer substrate 432, abackside grind/etch process is performed to remove the bulk substratelayer 400 of the semiconductor chip 420. For example, FIG. 4C is across-sectional schematic view of the package structure of FIG. 4B afterremoving the bulk substrate layer 400. In one embodiment, the bulksubstrate layer 400 of the SOI semiconductor chip 420 is removed using atwo-step grind and etch back process. In particular, a first stepincludes a mechanical grinding process to remove a majority of the bulksubstrate 400, followed by a selective chemical etch process to remove aremaining portion of the bulk substrate 400 selective to the BOX layer421. This process ensures that the BOX layer 421 is not over etched,which could occur if grinding is used alone.

After removal of the bulk substrate 400, a next process includespattering the metal layer on the surface of the interposer substrate 432to form contact pads. For example, FIG. 4D is a cross-sectionalschematic view of the package structure of FIG. 4C after patterning alayer of metallic material on the exposed surface of the interposersubstrate 432 to form contact pads 436 that are electrically connectedto the conductive vias 434 of the interposer 430, according to anembodiment of the invention. In one embodiment of the invention, thelayer of copper metallization on the surface of the interposer 430 ispatterned by depositing and patterning a photoresist layer to form aphotoresist mask, and then performing a metal etch process to etch themetallic material (e.g., copper) exposed through openings of thephotoresist mask and thereby form a pattern of contact pads/traces onthe surface of the interposer substrate 432.

After forming the contact pads/traces 436 on the surface of theinterposer substrate 432, the fabrication process continues with forminginverted pad structures which are used to electrically coupleoptoelectronic devices to the SOI semiconductor substrate 420. A processfor fabricating inverted pad structures will be discussed with referenceto FIGS. 4E, 4F, 4G, 4H, and 4I. For example, FIG. 4E is across-sectional schematic view of the package structure of FIG. 4D afterforming a photoresist mask 440 and etching a recess 442 in thesemiconductor chip 420 down to a buried pad 428 in the BEOL structure423 using the photoresist mask 440, according to an embodiment of theinvention.

More specifically, in one embodiment of the invention, a layer ofphotoresist material is deposited on the BOX layer 421, and thendeveloped and patterned to form the photoresist mask 440 as shown inFIG. 4E. One or more etch processes are then performed using thepatterned photoresist layer 440 as an etch mask to etch the portions ofthe BOX layer 421 and insulating layers of the BEOL structure 423 toform the recess 442 down to the buried pad 428 which is formed at someintermediate metallization level in the BEOL structure 423.

Next, FIG. 4F is a cross-sectional schematic view of the packagestructure of FIG. 4E after removing the photoresist mask 440 anddepositing a seed layer 450 which lines the exposed surfaces within therecess 442, according to an embodiment of the invention. The seed layer450 comprises a thin conformal seed layer which is formed using asuitable metallic material such as Ti, Cu, Ta, TaN, TiN, etc., anddeposited using known techniques such as CVD or PVD, etc.

Following deposition of the seed layer 450, the recess is filled with ametallic material to form an inverted pad structure. For instance, FIG.4G is a cross-sectional schematic view of the package structure of FIG.4F after depositing and patterning a layer of photoresist material toform a photoresist mask 460 which is used to cover the portion of theseed layer 450 on the BOX layer 421 and to expose the recess 442. Next,FIG. 4H is a cross-sectional schematic view of the package structure ofFIG. 4G after filling the recess 442 with metallic material 470 (e.g.,copper). Finally, FIG. 4I is a cross-sectional schematic view of thepackage structure of FIG. 4H after removing the photoresist mask 460 andetching back the metallic material 470 down to the BOX layer 421. A CMP(chemical mechanical polishing) process can be performed to planarizethe surface of the semiconductor substrate 420, e.g., etch down themetallic material 470 to be even with the BOX layer 421. The remainingmetallic material 470 forms an inverted pad (e.g., conductive vias 222,322, and 324 shown in FIGS. 2A, 2B, 3A and 3B).

Following the formation of the inverted pad structures 470, the contactpads 436 of the interposer substrate 432 are then bumped with C4 solderballs, or micro-C4, Cu pillars, etc., using a process that is commonlyimplemented in state of the art microelectronic packaging. For example,FIG. 4J is a cross-sectional schematic view of the package structure ofFIG. 4I after forming C4 solder balls 170 on the contact pads 436 of theinterposer 430, according to an embodiment of the invention. It isunderstood that part of the bumping process may include the depositionand patterning of an intermediate dielectric layer to form a mask for aplating process used to form the solder bumps 170.

Following formation of the solder bumps 170, layer of capping materialis deposited on the BOX layer 421 to provide an additional claddinglayer that prevents light from leaking out from silicon waveguides(e.g., silicon waveguide 426, FIG. 4A) that are formed from the activesilicon layer 422. The layer of capping material is patterned to form acapping layer (e.g., capping layer 142, FIG. 1) to expose portions ofthe BOX layer 421 and the inverted pad structures 470 to which externaloptoelectronic or photonic devices are bonded. Indeed, as discussedabove, exposing the inverted pad structures 470 allows optoelectronicdevices with corresponding ohmic contacts to be bonded to the invertedpad structures 470.

FIG. 5 schematically illustrates a high-level conceptual packageframework to implement an optical transceiver system, according to anembodiment of the invention. In particular, FIG. 5 schematicallyillustrates a package structure 500 comprising a high-performancemicroprocessor 505 coupled to a photonics communications system 510. Thephotonics communications system 510 comprises a SOI semiconductor chip520 comprising active analog/digital circuitry 530 to support opticalreceiver and transmitter functions. More specifically, the activecircuitry 530 comprises a SERDES (serializer/deserializer) circuit 531,a plurality (N) of transimpedance amplifier circuits 532-1, . . . ,532-N, a plurality (N) of laser driver circuits 533-1, . . . , 533-N, atemperature sensor circuit 534, a bias voltage generator circuit 535,and a phase-locked loop circuit 536.

Furthermore, a plurality of optoelectronic devices 540 are mounted tothe SOI chip 520, which include photodiodes 542-1, . . . , 542-N tosupport optical receiver functions, and laser diodes 544-1, . . . ,544-N to support optical transmitter functions. Each of theoptoelectronic devices 540 are coupled to a silicon waveguide structure550 comprising a silicon waveguide 552 and a vertical grating coupler554. The outputs of the photodiodes 542-1, . . . , 542-N are coupled tothe inputs of respective transimpedance amplifier circuits 532-1, . . ., 532-N. The outputs of the laser driver circuits 533-1, . . . , 533-Nare coupled to the inputs of respective laser diodes 544-1, . . . ,544-N.

The temperature sensor circuit 534 monitors the temperature of certainareas of the SOI chip 520 and generates sensor signals that are used byvarious circuits that are configured to have temperature-compensatedprogrammability, for example. The bias generator circuit 535 generatesthe requisite reference voltage(s) and/or reference current(s) that areused by the SERDES circuit 531, the transimpedance amplifier circuits532-1, . . . , 532-N, and the laser driver circuits 533-1, . . . ,533-N. The phase-locked loop circuit 536 generates a clock signal thatis used to sample the receive data and a clock signal that is used toclock the serial transmission of data.

To implement receive functions, optical data signals that are incidenton the vertical grating couplers 554 in one or more of the N receivepaths are captured by the vertical grating couplers 554 and transmittedto the inputs of the photodiodes 542-1, . . . , 542-N via the associatedsilicon waveguides 552. The photodiodes 542-1, . . . , 542-N convert thereceived optical data signals into electrical data signals in the formof a current. The transimpedance amplifier circuits 532-1, . . . , 532-Ncomprise current-to-voltage amplifiers which transform the current datasignals output from the respective photodiodes 542-1, . . . , 542-N intovoltage data signals that are processed by the SERDES circuit 531.

To implement transmit functions, serial data streams that are outputfrom the SERDES circuit 531 to the N transmit paths are input torespective laser driver circuits 533-1, . . . , 533-N. The laser drivercircuits 533-1, . . . , 533-N are configured to control modulation ofthe respective laser diodes 544-1, . . . , 544-N and cause therespective laser diodes 544-1, . . . , 544-N to generate and outputoptical laser signals that represent the data signals to be transmitted.The optical signals that are output from the laser diodes 544-1, . . . ,544-N are transmitted via the associated silicon waveguides 552 toassociated vertical grating couplers 554, where the optical signals arede-coupled and transmitted as light beams to a receiving opticalcircuit.

The photonics communications system 510 of FIG. 5 allows a transmit modein which data that is output from the microprocessor 505 on K×N parallelchannels is serialized into N parallel channels (K reduction) withincrease in the speed (bits/second) by a factor of K. Similarly, thephotonics system 510 allows a receive mode in which serial data streamsfrom N parallel channels are parallelized into K×N parallel channels (Kmultiplication) for input to the microprocessor 505 with a reduction inspeed (bits/second) by a factor of K. FIG. 5 shows an applicationexample where complex analog/digital can be integrated in a package withsome photonics functions allowing the integration of all the calibrationloops, which is required for high-yield reliable products.

FIG. 6 is a schematic cross-sectional side view of a package structureto implement an optical transceiver system based on the conceptualframework of FIG. 5, according to an embodiment of the invention. Inparticular, FIG. 6 illustrates a package structure 600 for packaging thehigh-performance microprocessor 505 and the photonics communicationssystem 510. In this embodiment, the high-performance microprocessor 505comprises a high-power density (>50 W/cm2) VLSI ASIC or servermicroprocessor. The high-performance microprocessor 505 is flip-chipmounted to one side of a package interposer 610 and the photonicscommunications system 510 comprising the optoelectronic devices 540 isflip-chipped to an opposite side of the package interposer 610, whichserves to thermally isolate the photonics communications system 510 fromthe high-power density of the VLSI microprocessor 505. Thehigh-performance microprocessor 505 is cooled using a heat sink 605mounted to the backside of the high-performance microprocessor 505.

The package interposer 610 is mounted to an application board 620,wherein a recess 622 in the application board 620 is formed to depththat allows the backside surfaces of the externally mountedoptoelectronics devices 540 (e.g., laser diodes, photodiodes) to makecontact to thermal vias 626 that are formed in the application board 620and provide cooling of the optoelectronics devices 540 to ensurehigh-reliability operation. In addition, a hole 624 is formed throughthe application board 620, and an optical fiber device 630 with afocusing lens 632 is aligned to the hole 624 to allow light beams 634 totransmit between corresponding optical fibers 630 and the verticalgrating couplers 554.

FIG. 7 is a schematic cross-sectional side view of a package structureto implement an optical transceiver system based on the conceptualframework of FIG. 5, according to another embodiment of the invention.In particular, FIG. 7 illustrates a package structure 700 for packagingthe high-performance microprocessor 505 and the photonics communicationssystem 510, wherein the high-performance microprocessor 505 comprisesmedium-power density (<50 W/cm2) VLSI circuitry. In this embodiment, thephotonics communications system 510 comprising the optoelectronicdevices 540 is flip-chip mounted directly to the active surface of thehigh-performance microprocessor 505, since the photonics communicationssystem 510 does not need to be thermally isolated from the medium powerdensity microprocessor 505. As further shown in FIG. 7, themicroprocessor 505 is flip-chip mounted to a package interposer 710,wherein the package interposer 710 comprises a hole 712 that is cutthrough the package interposer 710 to accommodate the photonicscommunication system 510.

The package interposer 710 is mounted to an application board 720. Theapplication board 720 includes a plurality of thermal vias 722 and aheat sink 724. The heat sink 724 is formed with a thickness such thatthe heat sink 724 contacts a backside of the optoelectronic devices 540(e.g., laser diodes, photodiodes) to cool the devices 540 and ensurehigh-reliability operation. In addition, a hole 726 is formed throughthe application board 720, wherein the optical fiber device 630 with thefocusing lens 632 is aligned to the hole 726 to allow light beams 634 totransmit between corresponding optical fibers 630 and the verticalgrating couplers 554. The package 700 of FIG. 7 enables shorterelectrical connections between the VLSI chip 505 and the electricalcircuitry and optoelectronic devices 540 of the photonics communicationsystem 510.

Although embodiments have been described herein with reference to theaccompanying drawings for purposes of illustration, it is to beunderstood that the present invention is not limited to those preciseembodiments, and that various other changes and modifications may beaffected herein by one skilled in the art without departing from thescope of the invention.

We claim:
 1. A method to construct a package structure, comprising:fabricating a first integrated circuit chip comprising asilicon-on-insulator (SOI) substrate, wherein the SOI substratecomprises a bulk substrate layer, a buried oxide layer disposed on thebulk substrate layer, an active silicon layer disposed on the buriedoxide layer, and a BEOL (back-end-of-line) structure formed over theactive silicon layer, wherein the active silicon layer comprises activecircuitry and an integrated optical waveguide structure; bonding a firstsurface of an interposer substrate to the BEOL structure of the firstintegrated circuit chip; forming conductive through vias in theinterposer substrate in alignment with contact pads of the BEOLstructure, and forming contact pads on a second surface of theinterposer substrate; removing the bulk substrate layer to expose theburied oxide layer; forming one or more inverted pad structures throughthe exposed buried oxide layer down to buried pads within the BEOLstructure; forming solder bumps on the contact pads of the interposersubstrate; and mounting an optoelectronics device to the firstintegrated circuit chip by bonding the optoelectronics device to ends ofthe inverted pad structures exposed through the buried oxide layer, andsuch that the optoelectronics device is aligned with a first end of theintegrated optical waveguide structure of the first integrated circuitchip; mounting the photonics package to a first side of a packageinterposer; mounting a second integrated circuit chip to a second sideof the package interposer, opposite the first side of the packageinterposer, wherein the package interposer comprises electrical wiringand through vias to provide electrical connections between the photonicspackage and the second integrated circuit chip; and mounting anapplication board to the package interposer, wherein the applicationboard has an integrated recess formed in one side of the applicationboard, wherein at least a portion of the photonics package is disposedwithin the integrated recess of the application board; and furtherwherein the application board comprises a plurality of thermal viasformed therein in alignment with the integrated recess, wherein thephotonics package is disposed within the integrated recess such that abackside of the optoelectronics device of the photonics package is inthermal contact with the plurality of thermal vias.
 2. The method ofclaim 1, further comprising bonding a waveguide core of a flexiblepolymer waveguide to portion of the buried oxide layer in alignment witha second end of the integrated optical waveguide structure, wherein thewaveguide core is adiabatically coupled to the second end of theintegrated optical waveguide structure through the buried oxide layer.3. The method of claim 1, wherein the optoelectronics device isadiabatically coupled to the first end of the integrated opticalwaveguide structure through the buried oxide layer.
 4. The method ofclaim 1, wherein first end of the integrated optical waveguide structurecomprises a vertical grating coupler to couple light between the opticalwaveguide structure and the optoelectronics device.
 5. The method ofclaim 1, wherein the optoelectronics device comprises at least one of alaser diode, a photodiode, a photonics device, and a combinationthereof.
 6. The method of claim 1, wherein the interposer substrate isformed of a material comprising one of glass, high-resistivity silicon,and an insulator material.
 7. The method of claim 1, wherein the activecircuitry comprises at least one of optical receiver circuitry andoptical transmitter circuitry, and wherein the BEOL structure provideselectrical connections from the active circuitry to the optoelectronicsdevice.
 8. The method of claim 1, wherein the second integrated circuitchip comprises a processor electrically coupled to the active circuitryof the first integrated circuit chip.
 9. The method of claim 8, whereincoupling the second integrated circuit chip to the first integratedcircuit chip comprises: flip-chip mounting the second integrated circuitchip to contact pads on a first side of an application board; andbonding the solder bumps on the contact pads of the interposer substrateto contact pads on the first side of the application board; wherein theapplication board comprises a plurality of high-speed transmission linesformed on the first side of the application board to enable electricalcommunication between the process of the second integrated circuit chipand the active circuitry of the first integrated circuit chip bonded tothe interposer substrate.
 10. The method of claim 9, wherein theinterposer substrate is formed of a material comprising one of glass,high-resistivity silicon, and an insulator material.
 11. The method ofclaim 8, wherein coupling the second integrated circuit chip to thefirst integrated circuit chip comprises using the solder bumps on thecontact pads of the interposer substrate to bond the interposersubstrate to an active surface of the second integrated circuit chip.12. The method of claim 11, wherein coupling the photonics package tothe second integrated circuit chip comprises: flip-chip mounting thesecond integrated circuit chip to contact pads a first side of anapplication board; and bonding the interposer of the photonics packageto contact pads on the first side of the application board; wherein theapplication board comprises a plurality of high-speed transmission linesformed on the first side of the application board to enable electricalcommunication between the processor of the second integrated circuitchip and the active circuitry of the first integrated circuit chip ofthe photonics package.
 13. The method of claim 12, wherein theinterposer includes at least one substrate having conductive vias andwiring to provide electrical connections to the BEOL structure.
 14. Themethod of claim 13, wherein the interposer substrate is formed of amaterial comprising one of glass, high-resistivity silicon, and aninsulator material.
 15. The method of claim 13, wherein coupling thephotonics package to the second integrated circuit chip comprisesbonding the interposer of the photonics package to an active surface ofthe second integrated circuit chip.
 16. A method to construct a packagestructure, comprising: fabricating a photonics package comprising: afirst integrated circuit chip comprising an insulating layer, an activesilicon layer disposed adjacent to the insulating layer, and a BEOL(back-end-of-line) structure formed over the active silicon layer,wherein the active silicon layer comprises active circuitry and anintegrated optical waveguide structure patterned from the active siliconlayer; an optoelectronics device mounted on the insulating layer of thefirst integrated circuit chip in alignment with a portion of theintegrated optical waveguide structure, and electrically connected tothe active circuitry; and an interposer bonded to the BEOL structure ofthe first integrated circuit chip, the interposer comprising at leastone substrate having a plurality of conductive through vias and wiringto provide electrical connections to the BEOL structure; and couplingthe photonics package to a second integrated circuit chip by flip-chipbonding the interposer of the photonics package to an active surface ofthe second integrated circuit chip, wherein the second integratedcircuit chip comprises a processor coupled to the active circuitry ofthe first integrated circuit chip; mounting the second integratedcircuit to a first side of a package interposer comprising the firstside and a second side opposite the first side with the photonicspackage disposed within a hole formed through the package interposerfrom the first side of the package interposer to the second side of thepackage interposer; and mounting an application board to the second sideof the package interposer.
 17. The method of claim 16, wherein theoptoelectronics device is adiabatically coupled to the integratedoptical waveguide structure through the buried oxide layer.
 18. Themethod of claim 16, wherein the integrated optical waveguide structurecomprises a vertical grating coupler to couple light between the opticalwaveguide structure and the optoelectronics device.
 19. The method ofclaim 16, wherein the optoelectronics device comprises at least one of alaser diode, a photodiode, a photonics device, and a combinationthereof.
 20. The method of claim 16, wherein the active circuitrycomprises at least one of optical receiver circuitry and opticaltransmitter circuitry, wherein the BEOL structure provides electricalconnections from the active circuitry to the optoelectronics device.